There are not-on-LRU pages which can be mapped and they are not worth tobe accounted. (becasue we can't shrink them and need dirty codes to handlespecical case) We don't want to account out-of-vm's-control pages.

When special_mapping_fault() is called, page->mapping is tend to be NULL and it's charged as Anonymous page. So avoid account it in __do_fault().We can know that by checking anon var.insert_page() also handles some special pages from drivers.